Electrical measurements having to do with the ion specificity of an excitable channel in the nerve membrane allows the experimenter to infer something about the molecular structure that is producing such a channel. Additionally, measurements of charged displacements that occur prior to the opening of the channel give independent information about the function of an excitable channel. The role of Ca in neurosecretion has been recognized for some time. It has been clear that the release of transmitter nerve terminals is a Ca-dependent reaction. Our studies of Ca entry and Ca removal from nerve fibers are focusing on the way that this ion may control electric excitation between cells as well as possible roles that Ca may have in the axonal membrane. To do this we are studying both pumping and the electrical entry of Ca as well as the way that Ca reacts with intracellular "buffers" so that its internal activity is kept low.